Saw a post (now deleted) about a watch that was rated for 30M being acceptable for swimming. Presumably because the author assumed that 30M meant the depth you can swim with it at? Or that because it was rated for 30M surely you can swim with it. The watch in question, the new Pebble, is rated at 30M or 3ATM.
Stop. Right now. This is where you get to learn about water resistance ratings.
10 Meters, 33 Feet, 1 ATM
A watch with this resistance level is protected against accidental exposure to water; for example, splashes, perspiration or accidental immersion. It should not be exposed to any water pressure.
30 Meters, 100 Feet, 3 ATM
This level will easily withstand splashes or brief immersion in water, or getting caught in the rain. However, it is not sufficiently resistant for swimming or bathing.
50 Meters, 165 Feet, 5 ATM
A watch with this level of resistance is wearable around household sinks, while playing sports and while swimming in shallow water. Do not wear it while bathing, snorkeling or scuba diving.
100 Meters, 330 Feet, 10 ATM
This level of water resistance will allow a watch to be worn around household sinks, while playing sports and while swimming, poolside diving or snorkeling. Do not wear it while scuba diving
150 Meters, 500 Feet, 15 ATM
A watch with this resistance level is wearable around household sinks, while playing sports and while swimming or poolside diving. It is also suitable for snorkeling and scuba diving.
Just because a watch is rated for 30 meters, that doesn't mean you get to go swimming with it. Not in 1 meter, not in 30 meters. This is based off of standing water pressure. The second you start moving your hand around you're generating more pressure. You'd be surprised the amount of pressure your body is capable of generating under water by moving.
30M, or 3ATM, is not a sufficient rating for swimming. Period. Most phones are 1ATM, this watch is 3ATM, you need 5ATM to go swimming. It's a standard, there's no room for "but the company said" or "but what if". If it's rated 3ATM, it's not rated to withstand swimming. If you go swimming with a watch rated at 3ATM and you damage it you have no one to blame but yourself. This watch is rated to withstand splashes of water, rain, possibly a shower.
Keep this in mind when buying anything you plan to wear in the ocean or anything else that's not more of a standing water than a pool: Oceans and such generate a lot of pressure on their own simply because the water is moving. Being rated to swim in shallow waters doesn't mean you can take this out in a rough ocean current.
When in doubt google the ATM rating the watch has and make an informed decision.